Field of the Invention
The present invention relates to an auto-focus apparatus and, more particularly, to an auto-focus apparatus suitable for reading an image on, e.g., a microfilm.
Related Background Art
A conventional apparatus for electronically reading an image on a microfilm will be described with reference to FIG. 1(A).
Light emitted from a light source a passes through a lens b and exposes a film 1. The light passing through the film 1 is selectively guided to a screen h or an image reader 3 by an optical path switching mirror d through a lens group c. Focusing is performed such that a focal point of the lens group c is adjusted by pivoting the mirror d toward the screen side X. Thereafter, the optical path is switched by pivoting the mirror d to the image reader side Y, and a CCD f is driven by a motor g. The intensity of light incident on the CCD f is converted into an electrical signal. Note that e in FIG. 1(A) indicates a mirror.
The correspondence between a focal point of an image focused on the CCD f and that focused on the screen h is adjusted by an adapter lens AL. This adjustment is performed during assembly in the factory. However, the correspondence may be lost due to vibration during conveyance or transportation, expansion, contraction, or change over time of the mechanism (optical path) due to temperature variations, etc.
Therefore, a service man readjusts the adapter lens AL to recover the correspondence after the apparatus has been delivered. However, this readjustment takes a long time and requires skill.
If the adapter lens AL itself fails to clearly focus a film image on the screen h, an operator cannot confirm if an optimally focused state (to be referred to as a "just-focus state" hereinafter) is obtained. Therefore, the adapter lens AL must be machined with high precision to minimize aberrations and distortion and to obtain very high resolution, resulting in increased cost.
In any case, even if the above requirements are satisfied, it is difficult for an unskilled operator to adjust an image to the just-focus state.
Note that the operator moves the lens group c vertically to vary the distance, thereby focusing on an image on the film 1. In this case, the lens group c is moved such that power from a manual control or a motor is decelerated by gears and the like. However, in the apparatus of this type, a vertical movement margin about the focal point is several micrometers to several tens of micrometers (corresponding to a focal depth). If the lens movement falls outside this margin, this results in an out-of-focus state, and image data cannot be read accurately.
Thus, the apparatus of this type, in which the focal point is adjusted by monitoring an image on a screen, has a large number of problems.
In recent years, a so-called electronic filing system using an optical disk, which files original images electronically, has attracted much attention. Users who conventionally record original images on microfilm to file them must convert the microfilm images into electronic data suitable for an optical disk. This can be achieved by an electronic microfilm scanner, an apparatus which has been developed increasingly.
However, as described above, an image on the microfilm must be focused manually for each frame of the microfilm, which is time-consuming and results in inconvenience.
In order to eliminate the above drawbacks, a method wherein a focusing lens can be adjusted in accordance with the thickness of a film at a point preset during adjustment in the factory, has been proposed.
However, the thickness of microfilms varies widely depending on the type (e.g., silver film, diazo film, and so on) and manufacturers. Therefore, with this method, films which can be used are inconveniently limited. In addition, the preset point will be shifted over time, and must also be periodically adjusted by a service man.
Meanwhile, in the still camera field, a large number of auto-focus mechanisms have been introduced. FIG. 1(B) shows a typical such auto-focus mechanism. Light passing through a lens U partially passes through a half mirror V, is reflected by a mirror W, and is then guided to a beam splitter T. The light is split into three beams respectively having three different optical path lengths by the splitter T. First to third sensors R1 to R3 detect respective focusing data. More specifically, they detect an amount of light and, the larger the amount, the better the beam is focused. When the amount of light detected by the second sensor R2 is the largest, this represents a just-focus state. When the amount of light detected by the third sensor R3 is larger than that by the second sensor R2, this represents a pre-focus state. When the amount of light detected by the first sensor R1 is larger than that by the second sensor R2, this represents a post-focus state.
In the case of, e.g., a still camera having a large focal depth (the distance of the just-focus state), the beam splitter can be used to change an optical path length, thus allowing easy detection of the focusing state. However, when the focal depth is very small (e.g., several micrometers to 20 micrometers, as in a microfilm reader), a beam splitter cannot be used due to physical limitations.
Since there are many types of films to be read or different densities in recorded images, if an identical auto-focus operation is performed with respect to all the films, a good focus state cannot always be achieved.
Since noise components from a motor or heater in the apparatus or outside may often be superimposed on outputs from image sensors that are the basis of the auto-focus operation, precise focus detection is interfered with.
As described above, the microfilm reader has many problems, and it is difficult to manufacture a microfilm reader with an auto-focus mechanism for practical application.